The cleaning and preparation of the silicon surface of a wafer for further processing is one of the most important tasks in the semiconductor industry. The main goal is to remove the contaminants from the wafer surface and to control chemically grown oxide on the wafer surface. Modern integrated electronics would not be possible without the development of technologies for cleaning and contamination control, and further reduction of the contamination level of the silicon wafer is mandatory for the further reduction of the IC element dimensions. Wafer cleaning is the most frequently repeated step in IC manufacturing and is one of the most important segments in the semiconductor-equipment business, and it looks as if it will remain that way for some time. Each time device-feature sizes shrink or new tools and materials enter the fabrication process, the task of cleaning gets more complicated.
Today, at 0.18-micron design rules, 80 out of approximately 400 total steps will be cleaning. While the number of cleans increases, the requirement levels are also increasing for impurity concentrations, particle size and quantity, water and chemical usage and the amount of surface roughness for critical gate cleans. Not only is wafer cleaning needed now before each new process sequence, but additional steps are often required to clean up the fabrication process tools after a production run.
Traditionally, cleaning has been concentrated in the front end of the line (FEOL) where active devices are exposed and more detailed cleans required. primary challenge in FEOL cleans is the continuous reduction in the defect levels. As a rule, a “killer defect” is less than half the size of the device line width. For example, at 0.25 μm geometries, cleans must remove particles smaller than 0.12 μm and at 0.18 μm, 0.09 μm particles.
Most cleaning methods can be loosely divided into two big groups: wet and dry methods. Liquid chemical cleaning processes are generally referred to as wet cleaning. They rely on combination of solvents, acids and water to spray, scrub, etch and dissolve contaminants from wafer surface. Dry cleaning processes use gas phase chemistry, and rely on chemical reactions required for wafer cleaning, as well as other techniques such as laser, aerosols and ozonated chemistries. Generally, dry cleaning technologies use less chemicals and are less hazardous for the environment but usually do not perform as well as wet methods, especially for particle removal.
For wet-chemical cleaning methods, the RCA clean, developed in 1965 by RCA, still forms the basis for most front-end wet cleans. A typical RCA-type cleaning sequence starts with a sulphuric-peroxide step (H2SO4/H2O2) followed by a dip in diluted HF (hydrofluoric acid). Next, an SC1 (Standard Clean step 1) uses a solution of NH4OH/H2O2/H2O to remove particles, followed by an SC2 (Standard Clean step 2) using a solution of HCl/H2O2/H2O to remove metals.
Despite increasingly stringent process demands and orders-of-magnitude improvements in analytical techniques, in the cleanliness of chemicals, and through the use of highly pure water, such as deionized (DI) water, the basic cleaning recipes have remained unchanged since the first introduction of this cleaning technology. Since environmental concerns and cost-effectiveness were not a major issue 30 years ago, the RCA cleaning procedure is far from optimal in these respects. Recently much research effort has been directed toward understanding the cleaning chemistries and techniques.
Important chemical savings can be obtained in an RCA-type cleaning sequence by using diluted chemistries for both the SC1 and SC2 mixtures. In the SC2 mixture, the H2O2 can be left out completely since it has been shown that strongly diluted HCl mixtures are as effective in the removal of metals as the standard SC2 solution. An added benefit of using diluted HCl is that at low HCl concentrations particles do not deposit, as has also been observed experimentally. This is because the isoelectric point for silicon and silicon dioxide is between pH 2 and 2.5. At a pH above the isoelectric point, the wafer surface has a net negative charge, while below it the wafer surface has a net positive charge. For most particles in liquid solutions at pH values greater than 2–2.5, an electrostatic repulsion barrier between the particles in the solution and the surface is formed. This barrier impedes particle deposition from the solution onto the wafer surface during immersion. Below pH 2, the wafer surface is positively charged, while many of the particles remain negatively charged, removing the repulsion barrier and resulting in particle deposition while the wafers are submerged.
To further lower the chemical consumption during wet wafer cleaning, some simplified cleaning strategies can be used, such as in a first step, organic contamination can removed and a thin chemical oxide grown. In a second step, the chemical oxide is removed, simultaneously removing particle and metal contamination. An additional third step can be added before final rinsing and drying to make the Si surface hydrophilic to allow for easier drying without the generation of drying spots or “watermarks.”
FIGS. 1A & 1B are illustrations of a single wafer cleaning chamber. FIG. 1A is an illustration of a wafer top loaded into the process chamber. FIG. 1B is an illustration of the top loaded wafer during processing within the single wafer cleaning chamber. An air filter can exist at the top of the process chamber and positioned beneath the air filter can be a wafer transfer slit. A rotatable wafer holding bracket can be extended to receive or release a wafer from a robot blade (not shown). The wafer must then be lowered by the bracket to a position below the air filter that is sufficient to avoid liquid spray from reaching the air filter. The wafer must also be positioned below a source for cleaning solutions onto the wafer such as a spray nozzle.
The composition and order of steps can vary for wafer cleaning but, during the cleaning process, all wafers are rinsed in pure water after each chemical immersion. The last few years have brought some changes such as with the use of megasonic energy to increase particle removal efficiency, but the basic cleaning philosophy used in most fabrication processes is still based on the original RCA process.
Megasonic agitation is the most widely used approach to adding energy (at about 800 kHz or more) to the wet cleaning process. The physics behind how particles are removed by such agitation, however, is not well understood. A combination of an induced flow in the cleaning solution, cavitation, the level of dissolved gases and oscillatory effects are all thought to contribute to particle removal performance.